Presently, low speed aeration rotors are large diameter centrifugal pump rotors that operate horizontally at the air-liquid surface boundary, mixing air and water. In use, the rotor draws water from beneath the rotor and sprays it horizontally over the water surface. The rotor also imparts a rotary motion to the body of water surrounding the rotor. In oxidation ditch applications, the rotary motion imparted by the rotor also forces the water in the ditch to circulate around the ditch. In one example of such a system, U.S. Pat. No. 3,510,110 to Kline, discloses an orbital system employing an elongated tank with central partition that includes a vertically-rotated surface aerator located at the end(s) of the partition wall for both aerating the sewage and circulating the sewage around the channels formed by the partition wall and the sides of the tank.
One example of an orbital system is sold under the trademark Carrousel®. An exemplified Carrousel® system, as with any typical oxidation ditch, has a basin that is shaped like a race track and has a central, longitudinally extending partition wall. The mixed liquor within the ditch is oxygenated by at least one low-speed vertical shaft aerator, which ensures proper mixing while generating the horizontal velocity and turbulence necessary to prevent sludge settling in the circuit. In use, while the wastewater is circulating around the channel, micro-organisms, such as activated sludge, utilize the organic compounds, nitrogen and phosphorus contained in the waste. Depending on how the system is employed, the circulation of the wastewater exposes the activated sludge to oxygen-rich, i.e., aerobic and oxygen-depleted, i.e., anoxic conditions. In use, the low-speed, vertical shaft, turbine aerator provides the necessary oxygen to support biological utilization, while also keeping the biomass in suspension by driving the wastewater in a turbulent flow across the entire looped channel. To obtain the most efficient level of nutrient removal, the power input is adjusted in relation to the actual oxygen demand and load conditions, by varying the speed and/or the submergence of each aerator. When the oxygen demand is low, aeration power can be further reduced by shifting the speed of the aerators, or by switching them off altogether.
The popularity of the conventional orbital systems is due primarily to their relative cost-effectiveness, simplicity of design, ease of operation and maintenance, and excellent effluent quality. The exemplified conventional orbital system can treat raw domestic water to EPA advanced secondary standards without primary clarifiers or effluent filters. With extended aeration, it produces a stable water sludge requiring little or no further processing prior to disposal. The conventional systems can be designed to have a power turn-down to match oxygen input to the mixed liquor to oxygen demand of the microbes acting to degrade the sewage, without loss of mixing and movement.
However, deep oxidation ditches and/or deep aeration basins (for example, and not meant to be limiting, about or greater than 4.5 meters deep) are sometimes beneficial because more matter can be processed in a given amount of surface area. However, the suction effect of conventional rotors is generally limited to about 6 meters in depth, and the rotary motion in oxidation ditches or basins is generally limited to about 4.5 meters in depth. In order to achieve a satisfactory flow velocity in the basins, conventional orbital systems are designed with a maximum depth of about 4.5 meters.
One example of a system for driving fluids below the effective depth limitations of the conventional rotors outlined above is disclosed in U.S. Pat. No. 4,869,818 to DiGregorio, et al. In this system, a radial flow submerged impeller is added to the same shaft that drives the surface aerator so that mixed liquor in the lower portion of the orbital channels is pumped in the same direction as that mixed liquor pumped by the surface aerator. Thus, the system urges movement of the fluid that would have been unaffected by the surface rotor and effectively alleviates certain depth restrictions in orbital tanks, which allows for the use of deeper channels that require less concrete and less land space. However, one will appreciate that adding an additional impeller that extends deep within the basin also requires additional power consumption.
In another example for providing movement of the fluid located near the bottom of aerated basins of greater depth, draft tubes are provided to cooperate with the surface aeration rotor. In this example, the draft tube, which is essentially a large diameter pipe, is fixed to and extends from the bottom of the basin such that its distal end is spaced a distance from the bottom of the basis and its proximal end is positioned below the surface aeration rotor. Here, the draft tube serves to concentrate the pumping action of the surface aeration rotor down toward the bottom of the aerated basin. However, the obstructive bulk of the fixed draft tube greatly attenuates the rotary motion imparted by the rotor, thus making the use of such a fixed draft tube impractical in a standard oxidation ditch and reduces the mixing effect in aerated basins. To overcome this limitation and to allow the use of a draft tube in a deeper ditch/basin system, conventionally practice requires at least one horizontal flow mixer that is positioned within the lower portion of the deeper ditch/basins. The additional required mixer requires more complex machinery and expense as well as increase the power consumption of the system.
From a dynamic point of view, the turbulent energy requirement of a fluid for proper mixing is related to physical properties of the fluid, turbulence length scale created by a particular agitating device and turbulent intensity which has dominant effect on rate of decay of kinetic energy. The turbulent intensity can be interpreted as fluctuating flow velocity and will affect the mass transfer of gas into liquid on gas-liquid interface. An energy efficient aeration method has to incorporate all these factors to produce the best mass transfer mechanism. Balancing of these physical phenomena produces the most energy economical aeration as well as to produce favorable flow configuration for good mixing and solid suspension. Moreover, for practical applications, maximum mechanical simplicity and minimum maintenance in operation are very important factors. The present invention is based on the above considerations.